1) Field of the Invention
The present invention relates to heat treating of structural members and, more particularly, relates to an apparatus and associated method for mechanically restraining structural members during induction heat treatments such as a stress relief cycle.
2) Description of Related Art
Residual stresses can result in structural members from various manufacturing and treatment processes. For example, if pieces of stock material are welded to form a more complex structural member, the member can include residual stresses that result from the welding process. These residual stresses can cause undesirable changes to the dimensional characteristics and material properties of the member. Conventional heat treatments are well known as a method of relieving stresses and thereby changing the mechanical and material properties of materials. For example, the structural member can be heated to a heat treatment temperature and then cooled. However, if the member is not mechanically restrained during the thermal cycle, the dimensions of the member may change during the heat treatment.
According to one proposed method of stress relief, tooling is positioned proximate to the structural member such that the tooling restrains the structural member. The structural member and the tooling are then heated in a furnace to the heat treatment temperature. The tooling restrains the structural member during the heating and subsequent cooling to maintain the dimensional accuracy. However, it can be difficult to provide tooling that is sufficiently strong and dimensionally accurate throughout the temperature range of the heat treatment cycle. Additionally, each structural member that is formed can require unique tooling for restraint during heat treatment, adding to the overall cost of the structural members. Further, even if such tooling can be provided, the process is time-consuming because it takes time for the furnace to heat the member and tooling to the heat treatment temperature. The time required for the subsequent cooling of the furnace, member, and tooling can also be lengthy.
Thus, there exists a need for an apparatus and associated method for heat treating structural members of various shapes and sizes. The apparatus should maintain the dimensional accuracy of the members during heat treatments such as a stress relief cycle. Preferably, the method should not be overly time-consuming.
The present invention provides an apparatus and method for heat treating a structural member, for example, to relieve stresses in the structural member. The structural member can be restrained during a heating and cooling cycle so that a desired dimensional accuracy is achieved. Further, structural members of various sizes and shapes can be restrained, and the heating and cooling cycle can be performed relatively quickly.
According to one embodiment, the apparatus includes first and second co-operable dies that are structured to define a die cavity therebetween for at least partially receiving the structural member. At least one susceptor is in thermal communication with the die cavity. Each susceptor has a Curie temperature at which the susceptor becomes paramagnetic, and the Curie temperature can be about equal to the heat treatment temperature of the structural member. An electromagnetic field generator, such as at least one induction coil, is configured to induce a current within at least a portion of the susceptors. A coolant source can be fluidly connected to the coils and configured to circulate a cooling fluid through a passage of the coils to cool the coils. At least one rigid tool is positioned in the die cavity proximate to the structural member. Each tool defines a surface corresponding to at least a portion of the structural member. Further, at least one bladder is positioned in the die cavity, each bladder configured to receive a pressurized fluid for expanding the bladder and thereby urging the structural member against the corresponding surfaces of the tools so that a distortion of the structural member is restrained while the structural member is heat treated.
According to one aspect, a pressure source is fluidly connected to the bladders to supply the pressurized fluid to the bladders. Two or more bladders can be positioned in the die cavity, and a pressure regulation device in fluid communication with each bladder can be configured to maintain a substantially equal pressure in each bladder. The bladders can also be configured opposite a portion of the structural member so that the bladders restrain the structural member therebetween, or the bladders can be configured between opposed portions of the structural member so that the bladders urge the opposed portions to a predetermined dimension. The bladders can be formed of titanium or titanium alloys.
According to another aspect, an inflatable susceptor engagement seal is disposed at an interface of first and second portions of the at least one susceptor and configured to be inflated to electrically engage the first and second portions. An inflatable cavity seal can be disposed at an interface of the first and second dies and configured to receive a pressurized fluid to inflate the seal to hermetically seal the die cavity.
The present invention also provides a method of heat treating a structural member. According to one embodiment, the method includes providing the structural member at least partially in a die cavity, positioning at least one bladder in the die cavity proximate to the structural member, and injecting a pressurized fluid into the at least one bladder and thereby expanding the bladder to at least partially fill a space in the die cavity and restrain the structural member in a predetermined configuration.
One or more tools are also positioned in the die cavity proximate to the structural member so that the structural member is urged against a corresponding surface of the tools. An electromagnetic field generator, such as at least one induction coil, is energized to induce a current within at least a portion of the susceptor to heat the structural member to a heat treatment temperature, such as a Curie temperature at which the susceptor becomes paramagnetic. A cooling fluid can also be circulated through the at least one induction coil. Thus, the structural member is restrained by the at least one bladder at least partially during the energizing of the coil so that the bladder restrains a distortion of the structural member. The structural member can be maintained at the heat treatment temperature for a predetermined interval to relieve stresses in the structural member. The structural member can also be cooled according to a predetermined temperature schedule while restraining the structural member with the bladders in the die cavity.
According to one aspect, at least two bladders are positioned in the die cavity, for example, opposite a portion of the structural member so that the bladders restrain the structural member therebetween. A substantially equal pressure can be maintained in each of the bladders. The bladders can be formed of titanium or titanium alloys. According to another aspect, an inflatable susceptor engagement seal is disposed at an interface of first and second portions of the susceptor and pressurized to electrically engage the first and second portions. The die cavity can be formed by engaging first and second cooperable dies, and an inflatable cavity seal at an interface of the dies can be pressurized to hermetically seal the die cavity. Gas can be purged from the die cavity, for example, before the bladders are expanded.
Before the structural member is placed in the die cavity, a fixture member that corresponds in shape to the structural member can be positioned in the die cavity. The bladders can be positioned in the die cavity proximate the fixture member and formed by heating the bladders to a forming temperature higher than the heat treatment temperature of the structural member and injecting a fluid to at least partially expand the bladders and urge the bladders at least partially against the fixture member. The fixture member is then removed from the die cavity. A forming susceptor having a Curie temperature about equal to the forming temperature can be provided in thermal communication with the die cavity, and a current can be induced in the forming susceptor to heat the bladders to the forming temperature.